Methods for achieving improved bond strength between unvulcanized and vulcanized rubbers

ABSTRACT

An improved method for achieving enhanced bond strength between components of unvulcanized and at least partially vulcanized rubber the improvement comprising the steps of selecting first and second initially unvulcanized rubber components (10, 12) for the manufacture of a vulcanized rubber article (21); applying an interphase layer (11) of rubber material, essentially devoid of crosslinking agents and containing from about 0.1 to about 4 parts by weight of at least one accelerator, per 100 parts by weight of rubber, to the first component; prevulcanizing the interphase layer and the first component together until both are at least partially vulcanized, establishing a gradient crosslink density (43) primarily in the interphase layer and thereby providing a lower crosslink concentration at the surface of the interphase layer opposite the first component; applying the second rubber component to the surface of lower crosslink density; and covulcanizing the components together, wherein the first and second unvulcanized rubber components and the interphase layer comprise rubber selected from the group consisting of natural and synthetic rubber and blends thereof. A similar improved method for achieving improved bond strength between components of unvulcanized and at least partially vulcanized rubbers the improvement comprising the steps of selecting two unvulcanized rubber components (10, 12) for the manufacture of a vulcanized rubber article (25, 35); treating one of the components under conditions that will establish a gradient crosslink density with a lower crosslink concentration at the surface (43); applying the other rubber component to the surface; and covulcanizing the components together.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. Ser. No. 08/344,474, filed Nov.23, 1994 now U.S. Pat. No. 5,645,674 which is a Continuation-in-Part ofU.S. Ser. No. 08/029,500, filed Mar. 11, 1993, now abandoned.

TECHNICAL FIELD

The present invention provides a method for improving or achieving goodbond strength between layers or plies of vulcanized rubber or partiallyvulcanized rubber and of unvulcanized rubber. One application for themethod is in the manufacture of rubber articles having multiple plylayers, at least two of which are contiguous, one being vulcanized i.e.,precured, while the other is unvulcanized.

A number of manufacturing processes, including the fabrication of fires,require or would benefit from the assembly and subsequentcovulcanization of precured and uncured rubber components. In theretreading of tires for example, in some instances precured treads areapplied to a buffed, used carcass with the aid of a thin, uncured rubberlayer. The radiation precuring of certain tire components has alsobecome a well-accepted commercial processing technique which allowsprecured and uncured rubber components to be interfaced.

BACKGROUND ART

Tires, conveyor belts and reinforced high pressure hoses are typical ofbut a few of the articles wherein cured and uncured components can becontiguously combined. Generally, the manufacture of these articlesinvolves the assembly of a plurality of layers of fully compoundedrubber that have been reinforced with carbon black and the like. Tires,as a more particular example, include components such as beads,sidewalls, carcasses, treads, and belts. In some instances, it may bedesirable to cure one or more of these individual components off siteand prior to assembly and vulcanization of the tire. An advantage ofprecuring is to impart integrity to the rubber based component so thatit will resist distortion during subsequent building and assemblyoperations which, in turn, allows more precise alignment of components,greater accuracy during building and, at the end of assembly, improvedtires. Also, because the bead and tread stock components have varyingthicknesses, by subjecting some such structural components to precure,the cure time of the final product can be decreased. Moreover, theability to combine vulcanized and unvulcanized rubber based componentswould permit a variety of articles to be manufactured, such as tires,utilizing one or more "standard" components or elements to whichvariable elements e.g., treads, can be bonded.

Accordingly, it has been desirable to precure or vulcanize certaincomponents either partially or fully prior to overall assembly toproduce the finished article. Unfortunately, however, the bond interfacebetween contiguous components, one of which is vulcanized and one ofwhich is unvulcanized, has not been acceptable. One manner of improvingthe adhesion calls for the mechanical buffing of the surface of thevulcanized rubber component, but this is an extra step and cannot alwaysbe employed.

The major hurdle to a broader application of this technique of employingprecured together with uncured components has been the lower adhesionobserved between the precured and the uncured compounds following theircovulcanization. The art has attempted to address the issue ofdeveloping or improving the bonding between contiguous rubber layers buthas not always been successful where the layers are cured and uncured.

U.S. Pat. No. 1,274,091, for example, discloses a composite sheet ofvulcanized rubber comprising sheets of uncured rubber that have beenwashed and dried, one of which is broken down by passage through rollsand contains a vulcanized agent, while the other is neither broken downnor contains a vulcanizing agent. The two uncured sheets are ultimatelycovulcanized.

U.S. Pat. No. 1,402,872 provides a method for uniting masses ofvulcanizable rubber by interposing a layer of rubber without sulfurtherebetween and then covulcanizing the multilayer mass.

U.S. Pat. No. 1,434,892 provides a method of forming a sheet of rubberby combining one ply containing sulfur with a second ply containing anaccelerator and thereafter covulcanizing the multilayer sheet.

U.S. Pat. No. 1,478,576 is directed toward sheet rubber patch materialsfor the repair of inflatable rubber articles. The material comprises acomposite including a rubber layer containing a non-migratoryaccelerator and a rubber layer containing sulfur.

Other U.S. patents which teach the covulcanization of uncured rubbersheets each containing different vulcanizing agents and/or amountsthereof include U.S. Pat. Nos. 1,537,865, 1,537,866, 1,569,662,1,777,960, and 2,206,441.

Thus, while others have covulcanized rubber sheets comprising differentvulcanizing agents and amounts, the art has not provided a method forattaining good adhesion between contiguous rubber articles orcomponents, one of which is vulcanized and one of which is unvulcanized.More particularly, the art has not recognized heretofore the existenceof gradient crosslink densities at the interface between cured anduncured rubber and hence, has not been able to provide good adhesiontherebetween.

The use of irradiation to effect a partial cure of at least a portion ofa tire component, other than the tread, followed by conventional cure ofthe tire is disclosed in U.S. Pat. Nos. 4,166,883 and 4,221,253 and4,851,063, respective divisionals. The foregoing patents do not suggestthe use of an interphase layer according to the present invention.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodfor bonding together and covulcanizing sheets of rubber or otherrubber-based components, one of which is at least partially vulcanizedand the other is unvulcanized, and to improve the bond strengththerebetween.

It is another object of the present invention to provide a method forbonding together sheets of rubber or other rubber-based components andto improve the bond strength therebetween, involving the use of aninterphase layer.

It is yet another object of the present invention to provide a methodfor bonding together sheets of rubber or other rubber-based componentsinvolving the use of an interphase layer that is devoid of crosslinkingagents such as sulfur, sulfur donors, peroxides, sulfur-less curativesand the like.

It is yet another object of the present invention to provide a methodfor bonding together sheets of rubber or other rubber-based components,one of which is at least partially vulcanized and having a gradientcrosslink density and the other is unvulcanized.

It is yet another object of the present invention to provide a methodfor bonding together sheets of rubber or other rubber-based components,one of which is at least partially vulcanized and the other isunvulcanized, which eliminates the procedure of mechanical buffing ofthe vulcanized rubber.

It is yet another object of the present invention to provide a methodfor bonding together sheets of rubber or other rubber-based partsinvolving the use of at least one chemical cure interfering agent.

It is yet another object of the present invention to provide a methodfor bonding together sheets of rubber or other rubber-based partsinvolving the use of irradiation to provide a gradient crosslinkdensity.

At least one or more of the foregoing objects, together with theadvantages thereof over known methods, which shall become apparent fromthe specification which follows, are accomplished by the invention ashereinafter described and claimed.

In general, the present invention provides an improved method forachieving enhanced bond strength between components of unvulcanized andat least partially vulcanized rubber the improvement comprising thesteps of selecting first and second initially unvulcanized rubbercomponents for the manufacture of a vulcanized rubber article; applyingan interphase layer of rubber material, essentially devoid ofcrosslinking agents and containing from about 0.1 to about 4 parts byweight of at least one accelerator, per 100 parts by weight of rubber,to the first component; prevulcanizing the interphase layer and thefirst component together until both are at least partially vulcanized,establishing a gradient crosslink density primarily in the interphaselayer and thereby providing a lower crosslink concentration at thesurface of the interphase layer opposite the first component; applyingthe second rubber component to the surface of lower crosslink density;and covulcanizing the components together, wherein the first and secondunvulcanized rubber components and the interphase layer comprise rubberselected from the group consisting of natural and synthetic rubber andblends thereof.

The present invention also provides an improved method for achievingimproved bond strength between components of unvulcanized and at leastpartially vulcanized rubbers the improvement comprising the steps ofselecting two initially unvulcanized rubber components for themanufacture of a vulcanized rubber article; treating one of thecomponents under conditions that will establish a gradient crosslinkdensity with a lower crosslink concentration at the surface to beadhered to the other component; applying the other rubber component tothe surface; and covulcanizing the components together, wherein thefirst and second unvulcanized rubber components comprise rubber selectedfrom the group consisting of natural and synthetic rubber and blendsthereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 3 schematically depict the covulcanization of two rubberbased components, one of which is prevulcanized, according to the priorart;

FIG. 4 is a cross-section of one layer of a rubber based component whichis unvulcanized;

FIG. 5 is a cross-section of the component depicted in FIG. 4, and theapplication of an interphase layer thereon, according to the presentinvention, both being subjected to prevulcanization;

FIG. 6 is a cross-section of the layers depicted in FIG. 5, with theapplication of a second unvulcanized layer of rubber based componentagainst the interphase layer and the final covulcanization thereof;

FIG. 7 is a cross-section of one layer of a rubber based component whichis unvulcanized and the application of an interphase layer thereon, bothbeing subjected to irradiation curing, according to another method ofthe present invention;

FIG. 8 is a cross-section of the layers depicted in FIG. 7, with theapplication of a second unvulcanized layer of rubber based componentagainst the interphase layer and the final covulcanization thereof;

FIG. 9 is a cross-section of one layer of rubber based component and theapplication of a chemical cure interfering agent onto a surface thereof,according to a method of the present invention;

FIG. 10 is a cross-section of the layer depicted in FIG. 9, with theapplication of a second unvulcanized layer of rubber based componentagainst the surface treated layer and the final covulcanization thereof;and

FIGS. 11 to 13 schematically depict the covulcanization of tworubber-based components and an interphase layer, as depicted in FIGS. 4to 6.

PREFERRED EMBODIMENT FOR CARRYING OUT THE INVENTION

As noted hereinabove, practice of the present invention allows thebonding of rubbers, of the same or different chemical composition, andof dissimilar states of vulcanization, in a facile manner to achieveimproved bond strengths.) The method is useful in the manufacture ofrubber goods from component plies or the like where it is desirable ornecessary to join one component that is unvulcanized with another thatis at least partially vulcanized.

Vulcanization involves crosslinking of the rubber molecules in thecomposition with the concentration of crosslink density governing thephysical properties of the vulcanizate. By partially vulcanized is meantthat the component is partially crosslinked and may be fully crosslinkedto the level desired to achieve complete vulcanization. Partialvulcanization is useful where green strength is too low and, partialvulcanization will impart enhanced physical properties, to withstandsubsequent manufacturing operations. Also, thicker rubber basedcomponents such as the bead area or the tread stock for tire manufacturemay be partially vulcanized in order to reduce final cure time of thefully assembled product. Partial vulcanization is also useful where thefinal product can be assembled by selecting a particular rubber basedcomponent from a plurality of different such components which is, inturn, combined with a "standard" component. In the manufacture of tires,for instance, a standard carcass could be built and prevulcanized forthe assembly of a variety of tires having different performanceproperties. By substitution of a treadstock, different tires could beproduced having the same standard carcass.

In any instance, as the rubber is prevulcanized, a problem that developsduring subsequent covulcanization of the components is that uponcompletion of the vulcanization a good bond has not developed at theinterface because the at least partially vulcanized component may becomeexcessively crosslinked at the interface, in effect, embrittling therubber at the interface.

The methods of the present invention solve these problems and providegood bond strength at the interface of the rubber layers or components.They do so by imparting a gradient crosslink density in theprevulcanized component such that the state of cure is lower at thesurface that is to become bonded to the adjacent layer of unvulcanizedrubber component in the ultimate covulcanization of the two components.

In one aspect of the present invention, a thin layer of rubber material,referred to herein as the interphase layer, is applied between thelayers or components of unvulcanized and at least partially vulcanizedrubber. Initially, the interphase layer is applied to an unvulcanizedrubber based component, both of which are subjected to prevulcanization.In another aspect of the present invention, the unvulcanized rubberbased component is subjected to an irradiation cure under conditionswhich produce a gradient crosslink density. Such means include, forinstance, high energy electrons and are useful because the degree ofcrosslink density can be varied through the thickness of the component.

In a third aspect of the present invention, the desired gradientcrosslink density is developed by initially applying a cure interferingagent to a surface of one of the rubber based components, that is tobecome at least partially vulcanized. The agent can be applied to theunvulcanized rubber component which is then subjected to cure conditionssufficient to impart at least partial vulcanization. However, at thesurface, little curing takes place while cure progresses within thethickness of the component. Alternatively, the agent can be applied to asurface of the rubber component subsequent to prevulcanization. Ineither instance, when the second, unvulcanized rubber based component issubsequently brought into contact with the formerly treated surface ofthe first component, a good bond is developed between the two componentsduring covulcanization as a result of the gradient crosslink densitybeing imparted due to the cure interfering agents.

Returning to the first aspect of the present invention, the compositionof the interphase layer is preferably substantially identical to thecomposition of the first rubber based component, for compatibility. Thecomposition of the interphase layer should also be compatible with thecomposition of the second rubber based component. By compatibility ismeant that the rubber selected to form the interphase layer is one whichwill allow the interphase layer to adhere well to the first and secondrubber components. Preferably, it will comprise a blend of the tworubbers used for the first and second rubber based components.

The interphase layer is preferably compounded without any crosslinkingagents, e.g., sulfur; sulfur donors, peroxides, sulfur-less curativesand the like. For purposes of the present invention, the termcrosslinking agent is employed in conjunction with compoundsconventionally known to vulcanize rubbers, both natural and synthetic.Typical examples include sulfur; sulfur donors, such as thiuramdisulfides and sulfur chloride; peroxides; sulfur-less curatives, suchas selenium and tellurium; polysulfide polymers; p-quinone dioxime;dibenzoyl-p-quinone dioxime; the metallic oxides, such as zinc, lead andmagnesium oxide; diisocyantes and the like. As is also known, suchcuratives can be employed alone to effect vulcanization or preferablywith accelerators, which are not to be considered as crosslinking agentsfor practice of the present invention.

Accelerators are nevertheless employed in the interphase layer inamounts ranging from about 0.1 to 4 parts per hundred of rubber (phr).Any of the conventional rubber accelerators can be employed such as theamines, thiurams, thiazoles, dithiocarbamates except for sulfur donorssuch as tetramethylethylenethiuram disulfide, sulfenamides andguanidines, it being understood that these accelerators are merelyillustrative and that practice of the present invention is notnecessarily limited to any specific accelerator, the presence thereofbeing merely optional. Moreover, while an accelerator may containsulfur, no elemental sulfur or other crosslinking agents are employed inthe interphase layer.

As will also be appreciated by those skilled in the art, acceleratorsare employed in conjunction with sulfur curatives, but not necessarilywith non-sulfur curatives, such as for instance, the peroxides and othercompounds disclosed hereinabove. Accordingly, in lieu of an accelerator,co-agents may be employed with the curative, one representative examplebeing the use of unsaturated monomers with peroxide curatives which,upon activation, copolymerize with the rubber polymer and therebycrosslink it. Reference herein to the presence of at least oneaccelerator in the interphase layer is intended to refer to thoseinterphase layer compounds that are sulfur curable while for compoundscurable by another system, the term accelerator shall include co-agents,as discussed herein.

Rubbers that may be combined via the present invention include naturalrubber and the synthetic rubbers. Synthetic rubbers are well known andinclude the ethylene/propylene copolymers, ethylene/propylene/dieneterpolymers, halogenated rubbers, copolymers of a conjugated diene withat least one monoolefin, conjugated diene homopolymers and mixturesthereof with and without natural rubber. Natural/synthetic rubber blendscan also be employed containing between about 5 to 95 parts by weightnatural rubber with the remainder being synthetic rubber.

Examples of suitable halogenated polymers include chloroprene,(2-chloro-1,3-butadiene or neoprene), chlorosulfonated polyethylene,chloro- and bromobutyl rubber. Neoprenes are generally categorized asG-types, W-types and T-types, each being well known to those skilled inthe art.

The copolymers may be derived from conjugated dienes such as1,3-butadiene, 2-methyl-1,3-butadiene-(isoprene),2,3-dimethyl-1,3-butadiene, 1,2-pentadiene, 1,3-hexadiene and the like,as well as mixtures of the foregoing dienes. The preferred conjugateddiene is 1,3-butadiene. Regarding the monoolefinic monomers, theseinclude vinyl aromatic monomers having from 8 to about 20 carbon atomssuch as styrene, alpha-methyl styrene, vinyl naphthalene, vinyl pyridineand the like and optionally one or more halogen substituents; alkylacrylates or methacrylates such as methyl acrylate, ethyl acrylate,butyl acrylate, methyl methacrylate, butyl methacrylate and the like;unsaturated nitrites such as acrylonitrile, methacrylonitrile and thelike and vinyl halides such as vinyl chloride, vinylidene chloride, suchas isobutene and the like and aliphatic monoolefins such as isobutene aswell as mixtures of the foregoing monoolefins. The copolymers maycontain up to 50 percent by weight of the monoolefin based upon thetotal weight of copolymer. A preferred copolymer is styrene-butadienerubber (SBR) a copolymer of a conjugated diene, especially butadiene,and a vinyl aromatic hydrocarbon, especially styrene.

The above-described copolymers of conjugated dienes and their method ofpreparation are well known in the rubber and polymer arts. Many of thepolymers and copolymers are commercially available. It is to beunderstood that practice of the present invention is not to be limitedto any particular rubber included hereinabove or excluded. The rubbershould be useful as a tire component although rubber compositions forother rubber articles may also be selected.

The compositions of the first and second rubber components will eachcontain a sufficient amount of one or more crosslinking agents, asdescribed hereinabove, to effect vulcanization when subjected to curingat conventional temperatures and times. One or more accelerators canalso be present in either or both first and second rubber components,again in a conventional amount for such compound and purposes. As willbe explained next, the crosslinking agents and optional acceleratorsfrom the first and second rubber components also assist in thevulcanization of the interphase layer.

According to the present invention, it is now possible to combine andbond components comprising the same rubbers e.g. natural rubber tonatural rubber or SBR to SBR or different rubbers, such as naturalrubber to SBR, either of which can be unvulcanized at the time of finalcovulcanization. Where one rubber component is to be at least partiallyvulcanized prior to contact with the second rubber based component,according to one aspect of the present invention, a thin interphaselayer is employed. Being substantially devoid of crosslinking agents,the interphase layer receives sulfur during prevulcanization bydiffusion from the adjacent rubber component (formerly unvulcanized)and, in certain instances accelerators where the adjacent rubbercomponent contained an accelerator. In this manner, a gradient crosslinkdensity is established, primarily in the interphase layer, as will bediscussed in conjunction with the drawing figures hereinbelow.Subsequently, when the at least partially vulcanized component is bondedto the other, unvulcanized rubber based component, a much greater bondstrength is developed than where at least partially vulcanized andunvulcanized rubber based components have been covulcanized heretoforewithout an interphase layer.

As previously noted, the interphase layer, devoid of crosslinkingagents, is preferably compounded from the same rubbers as the two rubberbased components. The interphase layer may also comprise a blockcopolymer compatible with the two rubber based components or othercompatible polymers. Generally, the composition and thickness of theinterphase layer can be optimized for interposition between the tworubber based components. Nevertheless, a thickness of at least 0.010inches is deemed to be practical, up to about 0.080 inches, althoughother thicknesses are not necessarily precluded. More particularly, athickness of about 0.030 inches up to about 0.045 inches is preferred.Generally, a thinner interphase layer will be employed where the sulfurdiffusion rate is low while a thicker interphase layer will be employedwhere the sulfur diffusion rate is high. Also, a thicker interphaselayer will be employed where the amount of accelerator is low or theactivity thereof is low while a thinner interphase layer will beemployed where the amount of accelerator is high or the activity thereofis high.

A preferred method for practice of the present invention is toprevulcanize together one unvulcanized rubber based component and theinterphase layer following which the second unvulcanized rubber basedcomponent and interphase layer of the prevulcanized product are broughtinto contact and subjected to covulcanization. Prevulcanization of theinterphase layer and first rubber based component is conducted for atime and temperature sufficient to cause or allow development ofphysical properties to a fraction of the optimum reached during completevulcanization. This fraction usually ranges from 10 to less than 100percent of optimum, but is not limited to this range.

It is to be understood that cure conditions will vary depending upon therubber compositions, their thicknesses and degree of vulcanizations, preand post cure treatment, and thus, no useful purpose will be served byspecifying a range of such conditions nor is such specificationnecessary. This is also true where the preferred method employsirradiation, as discussed hereinabove.

Where it is desired to employ an interfering agent, e.g., a cureretarder or cure deactivating agent, the interphase layer is notnecessary. Instead, the cure interfering agent is applied to a surfaceof the rubber based component that is to become or has been at leastpartially vulcanized. Typical cure retarders are well known to thoseskilled in the art including, for instance, salicylic acid, phthalicanhydride, benzoic acid, N-(cyclohexylthio)phthalimide,N-nitrosodiphenylamine, and others as are known to those skilled in theart. Typical cure deactivating agents include mineral acids. One methodof surface coating is to form a solution of the interfering agent in asolvent which can then be applied to the rubber based component in asuitable manner such as, but not limited to dipping, spraying, rollercoating and the like. It is to be understood that practice of thepresent invention is not limited to any particular cure interferingagent disclosed herein or to the method of application. Similarly, theamount utilized is not a limitation although, for example, amounts fromabout 0.1 to about 10 phr will be effective.

FIGS. 1 to 3 schematically show the concentration profiles ofcrosslinking agents and crosslinks formed in two rubber components, oneof which is prevulcanized, and one of which is unvulcanized, andsubsequently covulcanized together according to the prior art. In FIG.1, component 10 is prevulcanized and component 12 is unvulcanized.Component 12 has a concentration of accelerator 13 and a concentrationof sulfur 14. Component 10 has been prevulcanized with sulfur and anaccelerator and provides a crosslink density 15. During covulcanization,depicted in FIG. 2, the sulfur and accelerator from component 12 diffuseinto component 10, (area 16) contributing to the further crosslinkingthereof. After covulcanizing, as depicted in FIG. 3, the crosslinkdensity 19 of component 12 is lower near the interface 17 between thetwo components. In contrast, the presence of new crosslinks formed incomponent 10 during covulcanization (area 18), adding to the crosslinkdensity 15 formed during prevulcanization, cause the crosslink density19 to significantly increase near the interface of component 10. Thisregion of excessive vulcanization and embrittlement 20 results indecreased adhesion and possibly premature failure of the articlecontaining components 10 and 12.

Practice of the three aspects of the present invention is depicted inFIGS. 4-10. In FIG. 4, a layer of unvulcanized rubber based component10, depicted schematically, has been selected. In FIG. 5, the component10 has applied against it an interphase layer 11. These two layers arefirst covulcanized following which the unvulcanized layer 12 is applied.In FIG. 6, the three layers have been covulcanized together to form aproduct 21. In FIG. 7, a layer of unvulcanized rubber based component 10depicted schematically, also has applied against it an interphase layer11, both of which are together first subjected to irradiation viasuitable means 22, to develop a gradient crosslink density followingwhich the second, unvulcanized rubber based component 12 is applied. Theinterphase layer 11 is optional and when employed, it may optionallycontain one or more radiation inhibitors such as 2-naphthylamine;6-phenyl-2,2,4-trimethyl-1,2-dihydroquinone andN,N'-dioctyl-p-phenylenediamine in an effective amount, known to thoseskilled in the art. Such amounts range, for instance, between about 0.1to about 10 phr. Where no interphase layer is employed, the gradientcrosslink density occurs by virtue of the irradiation. In FIG. 8, thetwo layers have been covulcanized together to form a product 25.

Practice of the third aspect is depicted in FIGS. 9 and 10. A firstlayer of unvulcanized rubber based component 10 is treated by means 30which applies a coating of cure interfering agent 31 onto a surface 32of component 10. The component 10 is then prevulcanized sufficiently todevelop at least a partial vulcanization. The second unvulcanized layerof rubber component 12 is then applied against the surface 32 and thetwo components are covulcanized together, as depicted in FIG. 9 to formthe product 35.

FIGS. 11 to 13 show the concentration profiles of crosslinking agentsand crosslinks formed in rubber components resulting from practice ofthe method of the present invention, described in conjunction with FIGS.4 to 6. FIG. 11 again depicts the two rubber component layers 10 and 12with an interphase layer 11 therebetween prior to covulcanization. Theinterphase layer 11 is devoid of crosslinking agents, present only incomponent layers 10 and 12; however, one or more accelerators arepresent in the interphase layer. Component layer 12 has a concentrationof accelerator 13, and a concentration of sulfur 14. Component 10 hasbeen prevulcanized with interphase layer 11 and it will be noted thatthe crosslink density thereof 40 decreases at the interface 41 betweenthe two and continues to decrease across interphase layer 11. Thus, withthe diffusion of sulfur and accelerator during prevulcanization, agradient crosslink density is established across layer 11 providing asurface 43 of lower crosslink concentration.

In FIG. 12, the two components 10 and 12 are being covulcanized and itwill be noted that the sulfur and accelerator from component 12 diffuseinto interphase layer 11, and into component 10 (area 45), contributingto the further crosslinking thereof. In FIG. 13, it is evident that thecrosslinks formed during prevulcanization 40, together with thecrosslinks generated as a result of the sulfur diffusion occurringduring covulcanization (area 47) result in a more even crosslinkdistribution 46 throughout the composite and avoid the excessivevulcanization near the interface region of the prior art (FIG. 3).

In order to demonstrate the efficacy of the present invention, threerubber compounds were prepared which included a natural rubber compound(NR); a styrene butadiene rubber compound (SBR); and an interphase layer(IPL) comprising an SBR rubber and containing one accelerator. Thecompositions of these layers are presented in Tables I to III, with allnumbers representing parts per hundred rubber, based upon 100 totalparts by weight of rubber.

                  TABLE I                                                         ______________________________________                                        NATURAL RUBBER COMPOSITION                                                    STOCK TYPE: NR                                                                ______________________________________                                        Natural Rubber       100.00                                                   Carbon Black         62.00                                                    ZnO                  7.50                                                     Stearic Acid         0.50                                                     Adhesion promoter    0.88                                                     Paraphenylenediamine antioxidant                                                                   2.00                                                     Antioxidant          1.00                                                     Primary Sulfenamide accelerator                                                                    0.50                                                     Secondary Sulfenamide accelerator                                                                  0.30                                                     Sulfur blended in oil                                                                              6.25                                                     Phthalimide retarder 0.20                                                     TOTAL                181.13                                                   ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        SBR COMPOSITION                                                               STOCK TYPE: SBR                                                               ______________________________________                                        SBR                  100.00                                                   Processing oil       27.60                                                    Carbon Black         54.00                                                    ZnO                  2.00                                                     Stearic Acid         2.00                                                     Wax                  0.75                                                     Polymerized petroleum resin                                                                        3.50                                                     Paraphenylenediamine antioxidant                                                                   0.95                                                     Sulfur               2.25                                                     Sulfenamide Accelerator                                                                            0.60                                                     Sulfur donor         0.60                                                     TOTAL                194.25                                                   ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        INTERPHASE LAYER WITH ACCELERATOR                                             STOCK TYPE: SBR + primary accelerator                                         ______________________________________                                        SBR                     100.00                                                Processing oil          27.60                                                 Carbon Black            54.00                                                 ZnO                     2.00                                                  Stearic Acid            2.00                                                  Wax                     0.75                                                  Polymerized petroleum resin                                                                           3.50                                                  Diamine antioxidant     0.95                                                  Sulfur                  0.00                                                  Sulfenamide Accelerator 0.60                                                  TOTAL                   191.40                                                ______________________________________                                    

Peel adhesion studies were conducted utilizing the NR and SBR compoundswith and without the IPL interphase layer. Several combinations ofunvulcanized and vulcanized stocks were bonded together in the followingprocedure.

For testing purposes the rubber stocks to be adhered together were firstmilled to a thickness of 0.050 inch and cut into 6 inch squares. Rubberbacking material reinforced with polyester cords was also prepared tothe same dimensions. The first layer of backing was laid flat with thecords running horizontally. To this was applied a second layer ofbacking with the cords running vertically. One layer of test stock wasthen applied to the double backing layer thus completing one-half of theadhesion test pad. A second similar laminate was prepared for the secondtest stock intended for precuring, but it was also covered with a 6 inchsquare piece of interphase layer stock. The thickness of the interphaselayer was between 0.010 inch and 0.080 inch and its composition wasdescribed hereinabove. The interphase layer was then covered with a 6inch square piece of 0.006 inch polyester film. This film had either asmooth finish or a rough finish, as noted hereinbelow. (Where the testwas for checking the adhesion between two unvulcanized test stocks, theinterphase layer, polyester film, and prevulcanization step wereomitted.) The second laminate was prevulcanized with the interphaselayer in a positive pressure mold at approximately 278 psi for a timeand temperature which resulted in a prevulcanization ranging between 50and 100 percent of the optimum cure as determined by a shear rheometer.After prevulcanization, the laminate was demolded and the film wasremoved. Typical prevulcanization conditions for an SBR/interphaselaminate were 11 minutes at 165° C.

A 2 inch by 6 inch polyester separator film was placed over one end ofthe test stock in the first laminate and arranged perpendicular to thecords in the second backing layer closest to the test stock. Theprevulcanized laminate with the interphase layer was then laidface-to-face on top of the first laminate, with the cords in thebackings closest to the test stocks running parallel. The assembled padswere then covulcanized in a positive pressure mold at approximately 278psi at a temperature and time that was varied with the thickness of theinterphase layer in direct proportion but always at least five minuteslonger than the optimum cure time for the test stock in the firstlaminate, as determined by a rheometer cure curve. Typicalcovulcanization conditions for an SBR/interphase/NR laminate were 10minutes at 165° C. After demolding, one inch by six inch test stripswere die cut from the adhesion pad in such a way that the cords closestto the test stocks were parallel to the long axis of the test strip. Theseparator film was then removed and the test piece loaded into anInstron machine for 180° peel testing.

Peel adhesion values (lb/in) were determined for each of the bonded andcovulcanized laminates as follows: The six inch by one inch strips weregenerally tested at a clamp speed of two inches per minute. The strengthof the adhesive bond was measured in pounds per inch of sample width(typically one inch). At the start of the test the strength to initiatetearing at the adhesive interface reached a maximum and then was quicklyfollowed by a succession of smaller maxima and minima for the balance ofthe test. The first maximum at the start of the test was taken as thePeak value, and the smaller maxima and minima were averagedmathematically to arrive at the Plateau value. Two or more strips fromeach pad were tested, and the individual Peak and Plateau values wereaveraged to arrive at the final Peak/Plateau results.

Measurements were taken at 23° C. and 100° C. and the results have beenreported in Table IV for 12 separate examples as follows, comprising SBRas the unvulcanized or prevulcanized rubber component layers and,comprising SBR and natural rubber (NR) as the unvulcanized rubbercomponent layers. The SBR, NR and interphase layers were prepared fromcompositions provided in Tables I to III. Thicknesses were varied withappropriate adjustments in cure times.

EXAMPLE NO. 1

Two SBR layers, of a composition shown in Table II, were separatelylaminated to two polyester cord-reinforced backing layers and thenbonded together and covulcanized at 165° C. for 18 minutes.

EXAMPLE NO. 2

An SBR layer, laminated to two reinforced backing layers on one side anda smooth, 0.006-inch thick polyester film on the other side, wasprecured at 165° C. for 11 minutes. The polyester film was then removedand the freshly exposed rubber surface was bonded to and thencovulcanized with a laminate comprising an unvulcanized SBR layer andtwo reinforced backing layers at a temperature of 165° C. for 16minutes.

EXAMPLE NO. 3

An SBR layer, laminated to two reinforced backing layers on one side anda rough, 0.006-inch thick polyester film on the other side, was precuredat 165° C. for 13 minutes. The polyester film was then removed and thefreshly exposed rubber surface was bonded to and then covulcanized witha laminate comprising an unvulcanized SBR layer and two reinforcedbacking layers at a temperature at 165° C. for 16 minutes.

EXAMPLE NO. 4

An SBR layer, laminated to two reinforced backing layers on one side anda smooth polyester film on the other side, was precured at 165° C. for13 minutes. The polyester film was then removed and the freshly exposedsurface was first buffed and cleaned, and subsequently bonded to andcovulcanized with a laminate comprising an unvulcanized SBR layer andtwo reinforced backing layers at a temperature of 165° C. for 16minutes.

EXAMPLE NO. 5A

An SBR layer, laminated to two reinforced backing layers on one side andan interphase layer of 0.045-inch thickness on the other side, wasprevulcanized at 165° C. for 13 minutes. The composition of theinterphase layer was that listed in Table III. The polyester sheet,which covered one side of the interphase layer, was then removed and thefreshly exposed surface was bonded to and then covulcanized with alaminate comprising an unvulcanized SBR layer and two reinforced backinglayers at a temperature of 165° C. for 16 minutes.

EXAMPLE NO. 5B

An SBR layer, laminated to two reinforced backing layers on one side andan interphase layer of 0.045-inch thickness on the other side, wasprevulcanized at 165° C. for 13 minutes. The composition of theinterphase layer was that listed in Table III but without anyaccelerator. The polyester sheet, which covered one side of theinterphase layer, was then removed and the freshly exposed surface wasbonded to and then covulcanized with a laminate comprising anunvulcanized SBR layer and two reinforced backing layers at atemperature of 165° C. for 16 minutes.

EXAMPLE NO. 6

An SBR layer of a composition shown in Table II, and a NR layer of acomposition shown in Table I, were separately laminated to two polyestercord-reinforced backing layers. The two laminates were then bondedtogether and covulcanized at a temperature of 165° C. for 11 minutes.

EXAMPLE 7

An SBR layer, laminated to two reinforced backing layers on one side anda smooth, 0.006-inch thick polyester film on the other side, wasprecured at 165° C. to 11 minutes. The polyester film was then removedand a freshly exposed rubber surface was bonded to and then covulcanizedwith a laminate comprising an unvulcanized NR layer, of a compositionshown in Table I, and two reinforced backing layers at a temperature of165° C. for 10 minutes.

EXAMPLE 8

An SBR layer, laminated to two reinforced backing layers on one side anda rough, 0.006-inch thick polyester film on the other side, was precuredat 165° C. for 13 minutes. The polyester film was then removed and thefreshly exposed rubber surface was bonded to and then covulcanized witha laminate comprising an unvulcanized NR layer, of a composition shownin Table I, and two reinforced backing layers at a temperature of 165°C. for 10 minutes.

EXAMPLE 9

An SBR layer, laminated to two reinforced backing layers on one side anda smooth polyester film on the other side, was precured at 165° C. for11 minutes. The polyester film was then removed and the freshly exposedsurface was first buffed and cleaned, and subsequently bonded to andcovulcanized with a laminate comprising an unvulcanized NR layer, of acomposition shown in Table I, and two reinforced backing layers at atemperature of 165° C. for 10 minutes.

EXAMPLE 10A

An SBR layer, laminated to two reinforced backing layers on one side andan interphase layer of 0.045-inch thickness on the other side, wasprevulcanized at 165° C. for 13 minutes. The composition of theinterphase layer was that listed in Table III. The polyester sheet,which covered one side of the interphase layer, was then removed and thefreshly exposed surface was bonded to and then covulcanized with alaminate comprising an unvulcanized NR layer, of a composition shown inTable I, and two reinforced backing layers at a temperature of 165° C.for 11 minutes.

EXAMPLE 10B

An SBR layer, laminated to two reinforced backing layers on one side andan interphase layer of 0.045-inch thickness on the other side, wasprevulcanized at 165° C. for 13 minutes. The composition of theinterphase layer was that listed in Table III but without anyaccelerator. The polyester sheet, which covered one side of theinterphase layer, was then removed and the freshly exposed surface wasbonded to and then covulcanized with a laminate comprising anunvulcanized NR layer, of a composition shown in Table I, and tworeinforced backing layers at a temperature of 165° C. for 11 minutes.

                                      TABLE IV                                    __________________________________________________________________________    PEEL ADHESION VALUES - PEAK/PLATEAU                                                                               PEEL ADHESION (LBS/INCH)                  COMPONENTS IN LAMINATE              23° C.                                                                           100° C.                  EXAMPLE                                                                             COMPONENT 1                                                                            COMPONENT 2          PEAK                                                                              PLATEAU                                                                             PEAK                                                                              PLATEAU                     __________________________________________________________________________     1    SBR (Unvulcanized)                                                                     SBR (Unvulcanized)   226 >91C  111 >58C                         2    SBR (Unvulcanized)                                                                     SBR (Prevulcanized with smooth polyester                                                           198m)                                                                             134B   48 44B                          3    SBR (Unvulcanized)                                                                     SBR (prevulcanized with rough polyester film)                                                      --  --     53 46B                          4    SBR (Unvulcanized)                                                                     SBR (prevulcanized and then buffed)                                                                189 146B  --  --                           5A   SBR (Unvulcanized)                                                                     SBR (prevulcanized with interphase layer).sup.a                                                    264 226B  102 96B                          5B   SBR (Unvulcanized)                                                                     SBR (prevulcanized with interphase layer).sup.b                                                    208 190B  --  --                           6    NR (Unvulcanized)                                                                      SBR (unvulcanized)   212  71A   10 10A                          7    NR (Unvulcanized)                                                                      SBR (prevulcanized with smooth polyester                                                           110m)                                                                              22A   9   6A                          8    NR (Unvulcanized)                                                                      SBR (prevulcanized with rough polyester film)                                                      150  35A   7   7A                          9    NR (Unvulcanized)                                                                      SBR (prevulcanized and then buffed)                                                                168 >90C  --  --                          10A   NR (Unvulcanized)                                                                      SBR (prevulcanized with interphase layer).sup.a                                                    267 162B  153 98B                         10B   NR (Unvulcanized)                                                                      SBR (prevulcanized with interphase layer).sup.b                                                    100  66A  --  --                          __________________________________________________________________________     .sup.a accelerator present in interphase layer, no crosslinking agents        .sup.b no accelerator present in interphase layer, no crosslinking agents     A  Smooth Interfacial Tear                                                    B  Cohesive Tear at Interface                                                 C  Tear through test stock to reinforced backing                         

As is evident from Table IV, Example Nos. 5A and 10A, according to amethod of the present invention resulted in much improved adhesionvalues over Example Nos. 2 and 7 without any interphase layers as wellas the techniques involving the use of smooth, rough and buffedsurfaces. For purposes of comparison, Example Nos. 5B and 10B, employedinterphase layers but which did not contain any accelerators and it isreadily apparent that the bond formed between Components 1 and 2 was notas good, thereby establishing the efficacy of the present invention inthe use of interphase layers without crosslinking agents but withaccelerators. The plateau values for Examples No. 1 and 9 were reportedas greater than (>) the stated numerical value because tearing to thebacking occurred rather than tearing at the bond interface. Hence, thebond strength at the interface between the stocks was undetermined, butit is thought to be greater than the strength of the bond between thestock and the reinforced backing.

Based upon the foregoing disclosure, it should now be apparent that theuse of the methods described herein will carry out the objects set forthhereinabove. It should also be apparent to those skilled in the art thatthe methods of the present invention can be practiced to achieveimproved adhesion between a variety of rubber layers and componentsutilized in the manufacture of tires and other articles built from aplurality of plies or different components. Similarly, the time,temperatures and pressures for vulcanization can readily be determinedby those skilled in the art.

It is, therefore, to be understood that any variations evident fallwithin the scope of the claimed invention and thus, the selection ofspecific rubber compositions for the unvulcanized rubber based layers orcomponents as well as the composition and thickness of the interphaselayer can be determined without departing from the spirit of theinvention herein disclosed and described. Moreover, the scope of theinvention shall include all modifications and variations that fallwithin the scope of the attached claims.

What is claimed is:
 1. An improved method for achieving enhanced bondstrength between components of unvulcanized and at least partiallyvulcanized rubber, the improvements comprising the steps of:selectingfirst and second initially unvulcanized rubber components for themanufacture of a vulcanized rubber article, both said componentscontaining crosslinking agents; applying an interphase layer of rubbermaterial to said first unvulcanized component, said interphase layercontaining an effective amount of an irradiation inhibitor and beingdevoid of crosslinking agents that are capable of vulcanizing saidinterphase layer alone, and said interphase layer having a first surfacein contact with said first rubber component; subjecting said interphaselayer and said first component to a source of irradiation, whereby saidfirst component becomes at least partially vulcanized and a gradientcrosslink density is established in the interphase layer by themigration of crosslinking agents from said first component into saidinterphase layer, thereby providing a lower crosslink concentration at asecond surface of said interface layer opposite that in contact withsaid first surface of said interphase layer; applying said secondunvulcanized rubber component to said second surface; and covulcanizingsaid components together, said first and second unvulcanized rubbercomponents and said interphase layer being selected from the groupconsisting of natural and synthetic rubber and blends thereof.
 2. Amethod, as set forth in claim 1, wherein said first unvulcanized rubbercomponent is natural rubber said second unvulcanized rubber component isselected from the group consisting of synthetic rubbers and blendsthereof.
 3. A method, as set forth in claim 1, wherein said firstunvulcanized rubber component is selected from the group consisting ofsynthetic rubbers and blends thereof and said second unvulcanized rubbercomponent is natural rubber.
 4. A method, as set forth in claim 1,wherein said interphase layer comprises one or more rubbers selectedfrom the group consisting of natural and synthetic rubbers and blendsthereof.